A method of this type is known in particular from U.S. Pat. No. 4,407,667 published on Oct. 4, 1983 and claiming priority from French Patent Application FR 80 17005 filed on Jul. 31, 1980. The preforms are in the form of core rods designed to be built up while they are being displaced along the fiber-drawing axis, prior to being drawn into optical fiber. The core rods are butt welded, i.e. welded together end-to-end, by a welding station which is fixed relative to the fiber-drawing axis, and which comprises a furnace or an annular torch as a heat source for delivering the heat required for welding by melting the abutting ends of the two preforms. The two core rods are displaced along the fiber-drawing axis by rollers at the same speed of about 8 millimeters per minute.
The method described in that Application suffers from certain drawbacks.
Firstly, the displacement of the two preforms at a certain speed through the fixed welding station gives rise to heating of the preforms upstream and downstream from the two ends to be welded together. That surplus heat propagates axially along the preforms and contributes to extending the zone affected by the welding. When the preforms are drawn down, this zone often gives rise to an optical fiber that has degraded attenuation parameters, and that must be rejected from the output of the fiber-drawing method.
Secondly, the use of a furnace or of an annular torch as a heat source gives rise to pollution of the side surfaces of the preforms as they pass through the welding station.